This invention relates to an exhaust system for use with a watercraft propulsion arrangement. More particularly, the invention relates to an exhaust gas discharge system having one discharge path primarily for use during high speed vessel operation and another discharge path for use during both low and high speed vessel operation. The system is capable of discharging exhaust gases in a smooth manner, and is comprised of a relatively simple structure.
The treatment of exhaust gases generated during operation of watercraft propulsion units, and particularly outboard drives, is a troublesome one. With particular reference to inboard/outboard propulsion systems, an exhaust gas discharge system is known which discharges exhaust gases from an engine, disposed within the watercraft's hull, through an exhaust passage extending through a gimbal housing. The gimbal housing is secured to a rearward region of the watercraft's hull, and aids in supporting an outboard unit of the inboard/outboard propulsion system. For example, as disclosed in Japanese Unexamined Patent Publications Hei1-148695 and Hei1-204894, it is known to employ an exhaust passage which extends alongside an engine mounted within the watercraft's hull. This passage, in turn, leads to, and communicates with, a further exhaust passage which extends through a gimbal housing. This latter exhaust passage branches, at a branching section located within the region of the gimbal housing, into: (1) a pair of auxiliary passages which extend downwardly and terminate at outlet openings located along the bottom of the gimbal housing (for low speed/idle exhaust); and (2) a main exhaust passage which extends through the outboard portion of the inboard/outboard system and ultimately leads to an outlet opening formed through a central portion of an associated propeller boss (for normal/high speed exhaust).
In the systems just described, at the exhaust passage branching section located within the region of the gimbal housing, it has been the practice to utilize a construction wherein the combined sectional flow area of the pair of auxiliary passages is less than the sectional flow area of the main exhaust passage. In accordance with such construction, during low speed and idle operation of the watercraft the engine exhaust gases are discharged into the body of water within which the watercraft is being operated through the pair of auxiliary discharge passages. This is due to the fact that the outlets associated with these passages are positioned closer to the body of water's surface, during such operational conditions, than the outlet of the main exhaust gas discharge passage. On the other hand, when operating at higher speeds, the engine exhaust gases are discharged into the body of water via the outlet associated with the main discharge passage. Such discharge may be facilitated by a lower pressure region, or pocket, relative to the pressure existent within the exhaust system, which is generated behind the propeller boss as the propulsion unit moves through the body of water.
Such prior exhaust gas discharge arrangements which utilize a main underwater exhaust passage and an auxiliary discharge system structured to locate within a body of water at a position which is not as deeply submerged as the main exhaust passage have employed a construction wherein the total flow sectional area of the auxiliary discharge system is less than the flow sectional area of the main exhaust gas discharge passage. Thus, exhaust gases have been inhibited from passing through the auxiliary system during normal/high speed engine operation in such arrangements as a result of such relative flow sectional area dimensions.
Of course, upon utilizing an engine with a greater number of cylinders in such an inboard/outboard arrangement, the volume of exhaust gas produced during high speed operation can increase over that produced by engines having lower numbers of cylinders. With such an increase in the volume of exhaust gas produced, it becomes necessary to increase the flow area of the outlet opening of the main exhaust passage in order that the exhaust gases might be smoothly discharged. In order to achieve a through the hub exhaust outlet which possesses a larger flow area, it is necessary to increase the diameter of the propeller boss. Such an increase, however, creates certain problems. For example, an increase in the diameter of the propeller boss will, in turn, create an increase in the resistance to fluid flow around the propeller boss, and the lower casing region of the outboard unit, during movement of the associated watercraft across the body of water. This increase in resistance to fluid flow will increase the turbulence about the main exhaust outlet and will, thus, hinder the smooth discharge of the exhaust gases.
It is, therefore, a principle object of the present invention to provide an improved exhaust gas discharge system for a marine propulsion unit.
It is another object of this invention to provide an improved exhaust gas discharge system for use in a marine inboard/outboard drive.
It is yet a further object of the present invention to provide an exhaust system for a marine propulsion arrangement which is comprised of a relatively simple structure and which is capable of discharging a relatively high volume of exhaust gases in a smooth manner.
As set forth above, it is well known to discharge the exhaust gases from the powering engine through an underwater exhaust gas discharge (e.g., through the hub of a propeller) so as to utilize the body of water in which the watercraft is operating as a silencing medium. Although this is a very acceptable and effective way for silencing exhaust gases under high speed running conditions, it does present certain problems in connection with low speed exhaust gas discharge. Such problems include a high back pressure within the exhaust system due to the fact that the underwater discharge is generally relatively deeply submerged coupled with the relatively low exhaust pressure generated during such operation. With an outboard motor, it is the common practice to provide a separate, above the water, exhaust gas discharge which has its own silencing system for treating the idling exhaust gases. With inboard/outboard drives, on the other hand, the powering engine usually has a larger displacement and the treatment of the exhaust gases during idling presents different problems. As depicted in the above-discussed exemplary arrangements, it has been known to employ a further auxiliary exhaust gas discharge which is also underwater when the boat is traveling at low speeds but is less deeply submerged than the high speed exhaust gas discharge. The prior arrangements utilizing such an auxiliary exhaust gas discharge have been designed so that the auxiliary discharge has a smaller flow sectional area than the main underwater exhaust gas discharge, thereby preventing exhaust gases from passing through the auxiliary system during normal/high speed engine operation. Although generally these arrangements do provide good silencing, the low speed/idle exhaust gases tend to emanate in large bubbles which can cause objectionable noise.
It is, therefore, still a further object of this invention to provide an improved silencing arrangement for the exhaust gases of an inboard/outboard drive unit.